IB Biology ReviewDNA Transcription and Translation

What is Transcription?

The synthesis of mRNA from a DNA templateWhen the DNA nucleotides are copied into segments of RNATranscription occurs when a protein is needed by the body

The “blueprint” for the protein is copied from DNA

Transcription Vocab.

• Template Strand/Anti-sense Strand/Non-Coding Strand. The DNA strand that is a template for mRNA complementary base pairing during transcription

• Non-template Strand/Sense Strand/Coding Strand: The DNA strand that is complementary to the template DNA strand during transcription 

• Promoter Region: The region of DNA (a specific sequence of nucleotides) to which RNA polymerase binds to start transcription. (it signals the RNA polymerase where to begin translation along the DNA molecule)

• Terminator Region: The region of DNA that signals RNA polymerase to end transcription

• RNA processing: Removing the introns in pre-mRNA to produce mature mRNA that is ready for translation

• DNA Triplet: Three (3) DNA nucleotides such as ATA or GAC• mRNA Codon: Three mRNA nucleotides such as UAU or CUG

that were transcribed from the DNA triplet. Codons form the basis of protein translation because they code for specific amino acids that form proteins

What is Translation?

The synthesis of a polypeptide from the genetic information on a mRNA molecule when it couples with a ribosome. It occurs in 4 steps When the RNA segment tells the ribosome which amino acids to use to build a protein Translation turns the DNA code into a protein

RNA is the “blueprint” for the proteinThe ribosome follows the RNA’s directions

Translation Vocab.• tRNA Anticodon: The molecule that transfers/transports specific amino acids to the

ribosome where polypeptides • Polypeptides: These are the precursors of proteins and are the product of Translation.

They contain all the amino acids in the proper sequence, but the amino acid chain has not yet folded up into its final shape. After folding, carbohydrates are covalently bonded to the folded proteins in the Rough ER . Then, the molecule become a full-fledged protein and is sent to the Golgi Apparatus.

• Golgi Apparatus: This is where the proteins produced in the Rough ER are sorted and shipped mostly as secretory proteins used outside the cell such as insulin.

• Ribosomes: This organelle is the site of protein synthesis• Bound ribosomes: Ribosomes attached to the walls of the rough ER. Most extra-

cellular proteins which will be exported from the cell by exocytosis are synthesized on these ribosomes

• Free ribsomes: Ribosomes that are floating free in the cytoplasm. Most proteins used within the cell (such as the transport proteins in cell membrane) are synthesized on these ribosomes

• Polyribosomes: An aggregation of several ribosomes attached to one mRNA molecule. (see Figure 17.22). Polyribosomes speed up the translation of a protein.

• Point Mutations: A change in only one base pair of DNA in a gene• Substitutions: The replacement of one nucleotide and its complementary nucleotide

with another pair of nucleotides. Sometimes these types of mutations are silent mutations as when GAA mutates to GAG (they both code for glutamic acid. Others, such as the Sickle Cell Anemia Case Study described below, cause a change in one of the amino acids in a polypeptide

• Insertions and Deletions. These types of mutations lead to frameshift mutations and always produce proteins that are non-functional

What is the difference between DNA and RNA nucleotides?

DNA nucleotides areAdenineThymineGuanineCytosine

RNA nucleotides areAdeneineUracilGuanineCytosine

What are the steps of transcription?

1. RNA polymerase unwinds DNA helix2. Transcription starts at promoter region

3. RNA polymerase starts adding free RNA nucleosides to template strand of DNATranscription goes in a 5’ to 3’ direction

What are the steps of transcription? Cont.

4. RNA polymerase continues to add nucleosides

Nucleosides lose two phosphates as they bind to each otherBecome nucleotides5. RNA polymerase reaches terminator region6. RNA polymerase detaches from template strand7. RNA strand, now called mRNA, is released from DNA8. DNA double helix rewinds

The Three Steps of Transcription

1. InitiationRNA polymerase binds to helixRNA polymerase starts to add RNA nucleotides

2. ElongationRNA polymerase continues to add nucleotides

3. TerminationTranscription finishesRNA polymerase reaches terminator regionmRNA breaks offRNA polymerase detaches from helixHelix rewinds

What are the steps of translation?

1. mRNA binds to a ribosomeSmall sub-unit firstThen large sub-unit

mRNA translated in whatdirection?

5’ to 3’, like always

2. The first transfer RNA (tRNA) binds its anticodon to the start codon of the mRNAThe start codon is always AUG

Anticodon

Codon

tRNA

mRNA

Codons and Anticodons

Codon is complementary to the DNA strandAnticodon has the same sequence of nucleotides as DNA (except thymine is uracil)

64 different codons code for 20 different amino acidsThat means there are 64 anticodons that are attached to 20 different amino acids

Codons and Anticodons Sample Problem

The template strand of DNA is TAC-TTG-CTA-CAG-ATT

What will the mRNA (codon) sequence be?AUG-AAC-GAU-GUC-UAA

What will the tRNA (anticodon) sequence be?UAC-UUG-CUA-CAG-AUU

tRNA sequence is the same as the template strand, only with U in place of T

HL Only: tRNA Activating Enzymes

each amino acid has a specific tRNA-activating enzyme

called aminoacyl-tRNA synthetase

this enzyme binds the specific/correct amino acid to the correct tRNA

uses ATP for energy

HL Only: Ribosome Binding Sites

P site holds the tRNA carrying the growing polypeptide chainA site holds the tRNA carrying the next amino acid to be added to the chainDischarged tRNAs leave the ribosome from the E site

HL Only: DNA Sense and Antisense Strands

DNA sense strand / coding strand has the same base sequence as is as

mRNAExcept uracil replaces thymine

DNA antisense strand / template strand has the same base sequence as

Strand of DNA that is being transcribed (copied)

What are the steps of translation? cont.

3. Second tRNA binds to ribosome and mRNAEach tRNA brings a specific amino acid with it4. The large subunit moves the mRNA strand down after the second tRNA binds5. Amino acid on the first tRNA is bound to the second tRNA by a peptide bondProcess continues until ribosome reaches stop codon6. Then polypeptide (protein) is released

Genetic Code TableBe familiar with the genetic code tableBe able to use this to translate a segment of mRNA into a protein

Using the Genetic Code Table

The letters refer to the mRNA sequenceWhat is the amino acid sequence if the mRNA sequence is AUG-UCU-GCC-UUA-UCC-UACSequence is

Start/Met.SerineAlanineLeucineSerineTyrosine

HL Only: Free and Bound Ribosomes

Free ribosomes floating in the cytoplasm produce proteins that stay inside the cell (intracellular proteins)

Bound ribosomes on the rough endoplasmic reticulumproduce proteins that go outside the cell (extracellular proteins)

mRNA Processing

mRNA must be “processed” to remove what?Introns

What are introns?Non-coding segments of DNA

Why would introns have to be taken out?You wouldn’t want a protein made from them because they don’t do anything!

1. A certain gene codes for a polypeptide that is 120 amino acids long. Approximately how many nucleotides long is the mRNA that codes for this

polypeptide likely to be? (1 mark)

A. 30B. 40C. 360D. 480

Correct answer: C Each amino acid has three nucleotides

120 x 3 = 360

IB Exam Question

2. Distinguish between the structure of DNA and RNA. (3 marks)RNA

ribose (normally) single strandeduracilno double helix

DNAdeoxyribosedouble strandedthyminedouble helix

IB Exam Question

3. Explain the process of DNA Transcription including the role of the promoter region, RNA polymerase, and the terminator and 5’ to 3’ direction (7)

• Transcription is the synthesis of mRNA from a DNA template• It occurs in the 5’ to 3’ direction• It occurs in 3 stages: Initiation, Elongation, and Terminations• In Inititiation, RNA polymerase binds to the promoter region• RNA polymerase unwinds the DNA and begins adding

complementary RNA nucleotides to the template strand of DNA• In Elongation, the mRNA molecule lengthens• In termination, the RNA polymerase reaches the terminator

region and releases the mRNA molecule

IB Exam Question

4. Explain the process of translation.

(15 marks)• Translation consists of initiation, elongation, translocation and termination;• the binding of a ribosome to mRNA is called initiation;• the small ribosomal sub-unit attaches to mRNA, followed by the large subunit;• next, the first tRNA binds it’s anticodon to the start codon (AUG) of mRNA• This occurs at the A site of the large ribosomal subunit• each tRNA has an anticodon and the corresponding amino acid attached;• this is accomplished by a tRNA activating enzyme which links the correct amino acid

to each tRNA;• next, the first tRNA moves to the P site on the ribosome and a second tRNA binds to

A site • This is called translocation, the tRNA moves from the A site to the P Site after it

delivers its “payload” amino acid• The amino acid on first tRNA is bonded to amino acid on second tRNA;• this forms a peptide bond between the amino acids • the mRNA is translated in the 5' to 3' direction;• and the polypeptide lengthens (called elongation)• this process continues until the ribosome reaches a stop codon (called termination)• at this point, the polypeptide is released;

IB Exam Question

5. Compare DNA transcription with translation.

(5 marks)

• both in 5' to 3' direction;• both require ATP;• DNA is transcribed and mRNA is translated;• transcription produces RNA and translation

produces polypeptides;• transcription occurs in the nucleus (of

eukaryotes) and translation occurs in the cytoplasm at the rough ER;

• tRNA is needed for translation but not transcription;

IB Exam Question

6. What is removed to form mature eukaryotic mRNA?

(1 mark)

A. RNA primersB. ExonsC. RNA polymerasesD. Introns

Correct answer: D

IB Exam Question

7. Discuss the relationship between one gene and one polypeptide.

(1 mark)

• This means that each gene codes for one unique polypeptide

IB Exam Question

8. Describe the consequence of a base substitution mutation with regards to sickle cell anemia.

(Total 7 marks)• A mutation is a change in the DNA sequence;• This “mistake” often occurs during DNA

replication• This changes the mRNA during transcription;• Which, in turn, can change the amino acid

sequence;• A substitution mutation leads to a change to

one codon;• In sickle cell anemia, glutamic acid is changed

to valine / GAG to GTG;• This changes the shape of hemoglobin • The resulting hemoglobin cannot carry oxygen

as well;

9. Describe the Genetic Code.

(6 marks)• composed of mRNA base triplets;• These are called codons;• each codes for the addition of an amino acid to

a growing polypeptide chain;• the genetic code is degenerate;• this means that more than one codon can code

for a particular amino acid;• In other words, there are 64 different codons

that code for 20 amino acids• the genetic code is universal;• this means it is the same in almost all

organisms;